A desulfurization apparatus has been installed in a factory, a power plant, or the like to reduce sulfur content, such as sulfur oxide (SOX), contained in flue gas generated by combusting fossil fuel such as coal. Furthermore, from a viewpoint of reducing the operation cost for desulfurization, facilities such as a power plant adopt a method in which river water, sea water, or the like is utilized as an absorbent that is used for reducing sulfur oxide contained in flue gas (for example, see Patent Document 1).
In addition, to reduce costs, river water or sea water is used not only as an absorbent in the desulfurization apparatus, but also as makeup water for diluting the absorbent used in the desulfurization apparatus. FIG. 6 depicts an example of a gas purification system using river water as makeup water. As shown in FIG. 6, a gas purification system 100 includes: a sedimentation pound 104 that removes sediment and bulky materials 103 contained in river water 102 that is pumped up from a river 101; a screen 106 that removes plants 105; and a desulfurization apparatus 109 that brings sulfur oxide (SOX) contained in flue gas 107 into a gas-liquid contact with limestone gypsum slurry 108 to reduce SOX.
In the gas purification system 100, the river water 102 is pumped up by way of a pump 110, the sediment and bulky materials 103 are removed in the sedimentation pound 104. The plants 105 are then removed at the screen 106 to obtain makeup water 111. The obtained makeup water 111 is supplied to the bottom of an apparatus body 112. The makeup water 111 may be directly supplied to the bottom of the apparatus body 112; or the makeup water 111 may be used as limestone slurry by dissolving limestone therein, or as seal water for various apparatus, as well as makeup water for the desulfurization apparatus 109.
The limestone gypsum slurry 108 supplied to the apparatus body 112 is jet out of hydraulic nozzles 113 toward the top of the apparatus body 112, and is brought in gas-liquid contact with the flue gas 107 that is introduced from the wall side of the apparatus body 112, in the manner facing thereto. The limestone gypsum slurry 108 used for desulfurization of the flue gas 107 is formulated by mixing limestone slurry CaCO3 that is obtained by dissolving limestone powder in the makeup water 111, gypsum slurry CaSO4 that is obtained by reacting limestone with SOX contained in the flue gas 107 and then further oxidizing the resultant product, and water. For the limestone gypsum slurry 108, for example, liquid that is deposited in the bottom of the apparatus body 112 in the desulfurization apparatus 109 and is pumped up is used. SOX contained in the flue gas 107 reacts with the limestone gypsum slurry 108 in the apparatus body 112 in the manner shown in the following formula (1):CaCO3+SO2+0.5H2O→CaSO3.0.5H2O+CO2  (1)
The limestone gypsum slurry 108, containing SOX from the flue gas 107, is deposited in the bottom of the apparatus body 112, and mixed with the makeup water 111 and air 114, supplied to the bottom of the apparatus body 112, in the bottom of the apparatus body 112. In this process, the limestone gypsum slurry 108 flowed down in the apparatus body 112 reacts with the makeup water 111 and the air 114 in the manner shown in the following formula (2):CaSO3.0.5H2O+0.5O2+1.5H2O→CaSO4.2H2O  (2)
The flue gas 107 is purified, and released out from the top of the apparatus body 112 as purified gas 115. Liquid mixture 116 of the makeup water 111 and the limestone gypsum slurry 108 supplied into the desulfurization apparatus 109 is ejected out of the desulfurization apparatus 109, oxidized, and discarded as gypsum slurry into a pond 117, for example. The liquid mixture 116 is dehydrated and ejected out of the system as dehydrated cake (gypsum). The filtrate after dehydration (dehydrated filtrate) is subjected to waste water treatments, such as removal of suspended solids contained therein, pH adjustment, or removal of heavy metals contained therein. A part of the dehydrated filtrate is returned to the desulfurization apparatus 109, and the remaining dehydrated filtrate is treated as waste water.
In this manner, the gas purification system 100 ejects the limestone gypsum slurry 108 used in desulfurization performed by utilizing the river water 102 as the makeup water 111.